Tube bundle reactors comprise a plurality of parallel reactor tubes which are arranged in a tube bundle and into which catalyst material, usually of granular nature is filled. If desired, the reactor tubes also may be filled with inert material or variously combined and arranged catalyst and inert materials. The tube bundle is surrounded by a reactor shell, usually a cylindrical one. The ends of the reactor tubes are open and firmly positioned in tube sheets with their outer walls in sealing engagement with the tube sheets. The reaction gas mixture (feed gas) is supplied to the reactor tubes through a reactor head which spans the respective tube sheet. It is discharged as product gas mixture through another reactor head spanning the other tube sheet.
In a widely used system for dissipating heat of reaction generated in a tube bundle reactor during exothermal reactions a liquid heat carrier, such as heat carrier oil or a mixture of nitrate or nitrite salts, is passed through the reactor by circulation means so that it will flow around the outside of the reactor tubes, either in a concurrent flow or as a countercurrent to the flow of reaction gas mixture. Such a system is advantageous in that it offers operation which is practically pressureless and, as a consequence, the thickness of walls can be kept relatively thin even in big reactors. Suitable flow guide means ensure good and uniform dissipation of heat throughout the reactor cross section.
Another system of dissipating heat of reaction is by way of pressure fluid cooling, often performed as cooling by evaporation, such as cooling by boiling water. In this case the heat carrier system is operated under pressure, using fluids in forced or natural circulation that evaporate partly, fully, or not at all. The preferred fluid is water. As used here, the terms “boiling water cooling” and “boiling water reactor” are merely examples of “cooling by evaporation” and “tube bundle reactor with evaporative cooling”, respectively, and are not to be understood as restrictive.
With cooling by evaporation, the heat of reaction is dissipated by making use of it to evaporate part of the heat carrier. The resulting liquid-steam-mixture rises due to its low density and is passed out of the reactor to be split thereafter in a separator into a liquid phase and a steam phase. The liquid phase is returned to the reactor, while the steam phase can be utilized directly as working steam without any need for another heat carrier circuit. It is an essential advantage of this system that, at a given constant pressure, the temperature is the same everywhere in the heat carrier system as long a liquid heat carrier is present and the walls of the reactor tubes are in contact with the same. Moreover, very good heat transfer is obtained at the outer tube walls so that expenditure is negligible for flow improvement installations to enhance uniform distribution of the heat carrier. As the heat carrier evaporates in part only, the walls of the reactor tubes remain in constant contact with liquid heat carrier, whereby local overheating is avoided. Various embodiments of this type of structure are described, for example, in the following patents: U.S. Pat. No. 3,518,284, DE 2 013 297, DE 2 123 950, DE 2 420 949, DE 30 28 646, and EP 0 532 325.
For economic reasons, the aims with tube bundle reactors of the kind mentioned are to obtain large capacities, i.e. the greatest possible number of tubes and maximum useful tube volume to accommodate catalyst filling. Some reactor types require tube lengths in the order of 10 to 15 meters. Yet the desired capacity enlargement is subject to limitations imposed by the transportation environment. True, it has become possible by now to move relatively light apparatus with diameters of some 10 to 11 meters by road but, as a rule, shipment by road or rail of reactors equipped with tubes and having diameters of more than 4 to 5 meters over greater distances is impossible or forbids itself for reasons of cost. That is so, on the one hand, because of limited vertical and/or horizontal clearances or weight restrictions of bridges and the like and, on the other hand, the lack of suitable hoisting equipment at the place of reloading or the final site of erection.
Therefore, transportation of very big or very heavy reactors in parts to the construction site and assembly at the place of erection are desirable. At the same time, it is just as desirable to build the most compact tube bundle reactors which, however, still are such as to be suited for shipment in fully assembled state. As regards the first aspect, existing limitations of fabrication at a construction site make it imperative to determine the limits of weights and dimensions of individual reactor elements that will just stay within keeping of the limits imposed by transportation. In that way, the number of joints and seams can be kept as small as possible.
It should be noted also that it is very uneconomical and time consuming to equip the reactor with tubing at the construction site only, i.e. to thread the reactor tubes into the tube sheets and seal them to the tube sheets. In addition, in most cases it will not be possible to accomplish this at the site for lack of proper fabrication conditions and adequate quality assurance. Besides, the problems of dimensioning still would remain unresolved.
In the description below, designs will be discussed which relate to assembling a reactor. In particular, this will relate to reactors devised for assembly at the construction site or reactors composed of specifically defined subassemblies. Moreover, reactors will be described that are particularly compact and also reactor units that are combinations of a plurality of smaller reactors.
A tube bundle reactor to be put together at the construction site is specified, for instance, in DE 25 43 758 C3. It comprises at least two independent sectors with associated tube sheet sectors, outer shell sections, and inner wall sections including reactor tubes disposed inside the same, the respective walls of the sectors being mutually propped by spacer elements.
The individual sectors of such a tube bundle reactor can be tested for operability while still at the place of manufacture prior to being shipped because the reactor tubes will have been installed and the walls surrounding the reactor tubes define a tightly sealed space all around. It is at the erection site only that the sectors and the heads, the tube connections and the supports will be connected to one another, in the first place by welding procedures, to complete the tube bundle reactor.
This type of sectoral structure is suitable for operations in which the liquid heat carrier used is not under pressure and does not evaporate. The heat carrier space in this case is operated practically without pressure so that the thickness of the vessel walls essentially is determined by their own weight rather than by internal pressure. If such a tube bundle reactor were devised as a boiling water reactor, with a pressurized evaporating medium acting as heat carrier, the wall thickness of the reactor shell clearly would have to be greater for reasons of rigidity. Such a reactor is not suitable for the sectoral construction known from DE 25 43 758 C3. The main obstacle resides in the heat treatment after welding to which component parts with thick walls must be subjected in order to release internal tensions generated in the material by the welding process. Local heat treatment of the longitudinal connecting shell seams, that would be required with a boiling water tube bundle reactor according to DE 25 43 758 C3 delivered to the job site as several independent units having tubes already installed in them, is out of the question because of the differential thermal expansions between the reactor shell and the reactor tubes.
Further disadvantages of the sectoral construction are caused by the fact that the individual sectors dispose of quite a number of planar walls which either must be made unduly thick in view of the pressures prevailing inside the sectors or be reinforced by struts or some other kind of costly support. In addition, tension peaks caused by the internal pressure in the interior of the sector occur at the transitions between individual walls. Reactor tubes cannot be disposed in the central region of the reactor to utilize this region for reactions. Moreover, it must be sealed with respect to the upper and low reactor heads. The whole structure suffers from sealing problems and is susceptible to tension cracks, especially so if operated periodically and under frequently changing operating conditions, respectively.
EP 1 210 976 A2 describes the use of mechanical connector elements for assembling a tube sheet consisting of a plurality of tube sheet parts of a tube bundle apparatus. At the location of the joints, the tube sheet parts have complementary contours presented, for instance, as grooves and tongues which are secured to each other by pins extending through the grooves and tongues. To obtain a tight connection, the joint and the pin contours are welded together at both sides of the tube sheet. The essential advantages which this design is said to offer are smaller production machines, shorter production times, and clearly less distortion in comparison with fully welded connections throughout. Careful preparation is needed at the points of contact in such a structure. If the play is too great at the groove and tongue joint the transmission of force from one tube sheet part to another is reduced. If the play is too small pinching results and the parts cannot be joined. The bores to receive the connecting pins cannot be made until after the parts have been put together. Recesses must be formed at the counter-contours of edges so that these will not pinch. The tubes cannot be introduced and fixed in the tube sheet until after the connection has been completed. The fabrication process, therefore, must be performed at the manufacturing plant. Assembling the tube sheet parts at the job site makes little sense.
DE 1 667 187 C describes a solid bed, high pressure reactor for exothermal catalytic reactions with repeated intermediate cooling among a plurality of catalyst beds. The special feature of this reactor is a tube bundle evaporator serving as cooler. It is subdivided, arranged in the middle of the reactor, and fastened by its tube plate on the reactor head, preferably such that it can be pulled out. Having passed the reactor, the product gas exits from the reactor through a central opening in the tube plate of the first internal heat exchanger and a downstream outlet nozzle. The diameter of this type of reactor is limited due to the necessary thick walls which are needed because of the high process pressures. In this respect, therefore, a reactor described in the embodiment as having an inner diameter of 2.2 m presumably will present the upper limit. With diameters greater than that, a planar plate likewise could not be realized to serve as reactor head because of the great thickness needed. This would be even less feasible if, regardless of its shape, the reactor head were weakened by a plurality of passages for heat exchanger tube bundles. As diameters become greater, furthermore, a detachable flange connection becomes ever more problematic because of the size of the flanges and screws needed. Moreover, with greater diameters it remains unresolved how to transmit the weight load from the combination of catalyst bed and heat exchanger to the reactor shell. The requirements as to rigidity will not be met by a small projection provided within the reactor shell to serve as bearing surface, as illustrated in the embodiment, nor by the thin mounting plate at the top of the combination of catalyst bed and heat exchanger. The proposed structure is not suitable for final assembly at the construction site if the diameters of reactors are great.
DE 28 16 062 A1 describes a methanization reactor comprising a combination of solid bed and heat exchanger suspended in the reactor shell. Cooling of the reaction gas is effected exclusively by the cool feed gas entering. The entire weight load of the combination of solid bed and heat exchanger is accommodated by an upper mounting plate which is supported on and connected by screws to the upper end of the cylindrical reactor shell. A detachable screw connection interconnects the upper reactor head and the reactor shell. Product gas is led out of the reactor by an outlet duct which is sealed towards the reactor shell by a stuffing box. Compensation which would take care of different expansions in length of the cold reactor shell and the hot combination of solid bed and heat exchanger is not provided for. It is conceivable to disassemble the structure shown into its major elements, namely the reactor shell with the lower reactor head and the combination of solid bed and heat exchanger with the upper reactor head, and then ship them separately to the place of erection where they would be built together. The structure, however, is limited as to diameter for similar reasons as the solid bed, high pressure reactor known from DE 1 667 187 C. For this reason, separate shipments of individual parts to the job site and subsequent assembly there are not required. Moreover, the reactor has but one pressure space so that the design principle is not suited for application to a tube bundle reactor having separate reaction and heat carrier spaces.
Likewise known are tube bundle reactors whose tube bundle, mounted according to the floating head principle, can be taken out. The floating head principle provides for the tube sheet of one tube bundle end to be rigidly connected to the shell of the apparatus, whereas the other tube sheet is movable more or less freely. The tube bundle may be disposed either standing vertically upright or suspended, or it may be lying horizontally. A great number of variants of this design principle are specified in the respective standards, such as ASME, AD2000, or TEMA. The variants, in the first place, are concerned with how to direct the tube-side fluid out of the tube bundle. In the case of the free floating head, the tube-side fluid is deflected at the tube end so as to flow back in opposite direction. In its return flow, the tube-side fluid normally has a temperature which differs from the flow when entering and, therefore, compensation in length must be provided in the return line. The configuration of a U-tube bundle is an exception in that lengthwise compensation is effected by the U-tube bundle itself. Yet the tube-side fluid also may be passed on in the same direction through an outlet duct which leads out of the shell of the apparatus at the other end. Specifically in this case, lengthwise compensation must be provided at a suitable location.
DE 27 58 131 describes a tube bundle reactor which is used preferably for endothermal processes. The heat carrier, for example, is a heating gas which is guided around the reactor tubes by ring or disc-shaped deflector plates. The tube bundle is represented by a standing tube bundle with a floating head. An expansion joint is built into the connecting line between the floating head and the upper reactor head. The connecting line terminates at a flange in the upper reactor head, and a reactor outlet line is connected to this flange. The upper reactor head also is attached to the cylindrical reactor shell by a flange connection. A separate tube sheet head is connected to the lower tube sheet. A lower connecting line leads from the lower tube sheet head to the outlet nozzle of the lower reactor head and is welded to the same by its end. For exchange of the tube bundle, first the screw connections of the upper reactor outlet line and the upper reactor head are undone. Next, the welded connection of the lower connecting line is broken, whereupon the tube bundle can be withdrawn in upward direction. Upon completion of the maintenance or repair work, the reactor is reassembled in the opposite sequence. Although the reactor described is quite easy to dismantle, it still has the fundamental disadvantages of a floating head design. The tube side of the reactor is not sealed directly with respect to the surroundings but instead towards a second pressure space. That, of course, results in a more expensive structure, and the interior pressure envelope suffers additional strain from the surrounding medium and the pressure and temperature thereof.
A tube bundle reactor having a floating head implemented in suspended form is known from U.S. Pat. No. 5,006,131 B. It forms part of the reactor system described in that publication. An upper reactor head having gas outlet nozzles is connected to the edge of the upper tube sheet. The upper reactor head further includes a manhole nozzle. A cylindrical shell connected in downward direction to the edge of the upper tube sheet merges at its lower end into a lower reactor head. At their lower ends, the reactor tubes open into a floating head consisting of a lower tube sheet and a lower tube sheet head. An outlet line leads from the lower tube sheet head through the lower reactor head and out of the reactor. The outlet line is connected to the lower reactor head by an expansion joint located outside of the reactor. A heat carrier enters the reactor at one side and leaves it at the other, while ring or disc-shaped deflector plates guide it as it flows through the reactor, always vertically with respect to the tubes in order to enhance the heat transfer. This type of reactor is not suited for final assembly at the site of its erection because several thick sheets must be welded together. The preferred number of tubes is small, being indicated as 250 and 1000 in U.S. Pat. No. 5,006,131 B. Additionally, the observations made in the discussions above of DE 1 667 187 C regarding the problems of greater reactor diameters and of DE 27 58 131 regarding the disadvantages of a floating head design apply here as well.
DT 25 13 499 A1 describes a reforming oven which functions by heat convection and a system provided with such an oven to produce gas rich in hydrogen or synthetic gas. In principle, the reforming oven or reactor may be used in addition for many other applications. In the case of the embodiment specified, the reforming oven, referred to below as reactor, includes a multipart tube sheet to which a bundle of reactor tubes are attached. At their lower ends, the reactor tubes open into a central pipe which extends upwardly and further on through the upper reactor head to the outside of the reactor. The tube sheet, including the reactor tubes suspended from it, is fixed radially inwardly by a strong welded connection to the central pipe through which the product gas leaves the reactor, and at its outer peripheral edge it is connected by an elongate carrying member to the lower part of the upper reactor head. The elongate carrying member is relatively elastic and thus able to compensate differential radial expansions of the cylindrical shell, upper reactor head, and upper tube sheet. Such movements are intended to be reduced by an insulating layer within the multipart upper tube sheet. Since the upper reactor head is flange-connected to the major cylindrical reactor part it is easy to pull the upper reactor head out of the reactor, along with the tube sheet and reactor tubes. The design described is suitable only for small and medium reactor diameters because the entire tube bundle is suspended mainly by the central pipe. The elongate carrying member between the tube sheet and the upper reactor head can accommodate only minor forces. Separate shipment and later joining of individual reactor groups at the job site would seem to be conceivable. But to fill the tubes with granular catalyst material and, above all, to empty them again would be difficult, if at all to be fully accomplished.
A structure designed for multiple length compensation is known from EP 1 048 343 A2 which describes a tube bundle reactor having the upper ends of the reactor tubes fastened individually by expansion joints to an upper tube sheet. The lower ends of the reactor tubes are firmly connected to a lower tube sheet. The lower tube sheet is enclosed by a head to which a nozzle is connected. This nozzle in turn (at its other end) is connected to the lower reactor outlet nozzle by an expansion joint which is disposed still inside the reactor. The welding must meet high demands where an individual reactor tube is to be connected to a tube sheet with the aid of an expansion joint. It makes sense only to accomplish this kind of work at a manufacturing plant. Fabrication at the place of erection practically is excluded. Also, dividing the tube bundles including the tube sheets secured to them causes problems since the expansion joints are highly sensitive and could be damaged during transportation.
With the aim to provide a compact reactor unit, it is suggested as a possibility in EP 1 590 076 A1, for example, that on entering a tube bundle reactor devised for partial evaporation of a heat carrier, the latter be distributed uniformly around the reactor circumference by interior annular channels. In this way the number of conduits outside of the reactor is reduced to a minimum. But still, nozzles are needed radially at the circumference of the reactor shell and, as a consequence, the overall dimensions of the reactor for shipment are enlarged. This type of reactor is not suitable to be dismantled into several subassemblies for shipment.
DT 1 542 494 C3 describes a tube bundle reactor in which liquid salt is used as heat carrier. The heat carrier gives off heat which it has taken up in the tube bundle reactor through laterally protruding conduits in a heat exchanger disposed outside of the reactor, e.g. embodied by a steam generator, to be returned later into the reactor. A particular characteristic of this tube bundle reactor is the arrangement of the reactor tubes in a plurality of tube bundle sectors between which there are passageways which are free of tubes. One passageway is somewhat larger than the others so that, in addition, it can house an inlet line to the heat exchanger and an outlet line from the heat exchanger. A guide duct including a built-in impeller is disposed centrally in the tube bundle reactor and driven by an electric motor. The major part of the heat carrier conveyed downwardly by the impeller enters the space between the lower tube sheet and an orifice plate located above the same and is then distributed uniformly throughout the entire reactor cross section by means of the passageways and the orifice plate. This major amount flows upwardly around the reactor tubes and, having passed through an upper orifice plate, reenters the guide duct at the top. The residual amount of heat carrier flows through an inlet line into the heat exchanger where it gives off the heat it had taken up and flows back to the tube bundle reactor through a return line into the inlet of the central guide duct. A remarkable detail of this design is the distribution and collection, respectively, of the heat carrier centrally in the interior of the reactor, whereby expensive valve means, annular channels, and specifically devised windows for entry into the interior of the reactor can be dispensed with.
WO 2004/004884 A1 suggests a reactor system and a reactor arrangement, respectively, in which a plurality of reactor units are operated in parallel like a single reactor, dispensing with individual measuring and control means. These reactor units are operated with a common heat carrier system, preferably applying boiling water cooling in a natural circulation layout. The reaction gas is supplied through one or more conduits to two or more reactor units. For the product there is one withdrawal or several ones in common. It is preferred to use tube bundle reactors, the reactor tubes being filled with catalyst. The reactor system presented in the document offers a solution for achieving the efficiency of a very big reactor and profiting from the transportability of the individual reactors for assembling the reactor system.
It is, indeed, a widely applied principle in process engineering to connect several smaller units in parallel to obtain one bigger unit. As a rule, however, this has the disadvantage of requiring expensive distribution and collection systems and the associated piping and nozzles for supply and discharge of the reaction gases and heat carrier fluids to and from the individual units. Furthermore, each one of the smaller units must provide accessibility toward its interior, such as for catalyst replacement. Connections in parallel also involve extra expenditure caused, for instance, by supporting of the units, mutual compensation of thermal expansions, insulation, space requirement, and the surrounding steel structures. No measures are indicated in WO 2004/004884 A1 for minimizing the additional expenditure involved with connections in parallel so that it might be possible to come up with solutions which are economical.
In chemical production plants on an industrial scale it is often desired, for economic reasons, to have one big tube bundle reactor unit with as many reactor tubes as possible. But the dimensions and/or weight of big tube bundle reactors surpass the limits of transportability. Known designs of subassemblies are not suitable for tube bundle reactors with pressure fluid cooling. Besides, it is desirable, again for reasons of shipment but also for ease of assembly, that such tube bundle reactors or tube bundle reactor arrangements be compact in structure and able to function without a lot of accessory equipment.